Are you ready to learn some aerodynamic theory? The four forces of flight are essential in understanding how an airplane flies and how it behaves. A firm grasp of these concepts is the foundation of nearly all your flight training, both in theory and practice. Today I will describe the four basic forces of flight and how they affect the aircraft.
What are the Four Forces of Flight?
The four forces of flight are lift, drag, thrust, and weight. At any given stage of flight, these forces all interact together to produce a given response in the aircraft. Lift and weight are opposites, as are thrust and drag, but each force also has a bearing on the other three.
Here’s what you need to know.
The Four Forces
Lift
Lift is the main force that acts on the aircraft wing and enables airplanes to fly.
How does it work, and how is lift generated?
If you take a quick look at a wing in cross-section, you’ll see that the upper surface is curved. This is called camber. A curved surface is longer than a flat surface, so it takes air longer to travel over the top of the wing than the bottom.
This creates a pressure differential, with low-pressure air on top of the wing and higher-pressure underneath. The wing will be subject to a force resulting from this differential, and this force is called lift. Suppose you want to read more about the causes of this phenomenon. In that case, I’d strongly advise you to check out Bernoulli’s Principle.
Here are the things that affect the amount of lift produced by a wing:
- The camber of the wing
- The size of the wing
- The speed of the wing through the air
- The angle of attack
If you increase any of the above four things, then the wing will produce more lift. (Angle of attack is slightly unusual in this regard. See my article on stalling to understand what happens with excessive angles of attack).
Lift always acts at right angles to the upper surface of the wing. Remember this concept. It is important.
Drag
In essence, drag is air resistance. If you have ever walked into a stiff breeze, you have experienced just a tiny bit of drag. There are actually several types of drag. Drag acts in a rearward direction and is the opposing force to thrust.
Here are the types of drag:
- Parasite Drag – This is caused purely by the shape of the aircraft and what its surfaces are made from. Smoother surfaces have less parasite drag, rougher surfaces generate more. The shape makes a huge difference too. A truck makes more drag than an indycar, and the same principle applies to aircraft.
- Induced Drag – Induced drag is a little unusual because it is made as a byproduct of lift. Without getting too complex, not all of the air goes over and under the wing. Some air escapes around the wingtips, where it mixes and forms vortices. These vortices hurt aircraft performance.
Thrust
Thrust is the force pulling the aircraft forward. It can be achieved normally using the aircraft engine.
Thrust always acts in the direction the aircraft’s nose is pointing.
Remember how we talked about ways to generate more lift?
If the thrust outweighs the drag, then the aircraft will accelerate and fly faster… This leads to more air passing over the wing, and therefore more lift! If drag exceeds thrust, then the aircraft will slow down.
Weight
Weight is the combined force of gravity acting on the entire aircraft. It includes everything onboard the aircraft, such as:
- The airframe
- The engine
- The fuel and oil
- The bags
- And, of course, you, the pilot
These weights are all added together and are focused through a single point, called the center of gravity.
Weight is the opposing force to lift. However, it is important to understand a key concept.
Whereas lift always acts at a 90-degree angle to the upper wing’s surface, the weight always acts towards the geometric center of the earth.
Meaning?
Regardless of which direction the aircraft faces, whether up, down, or even inverted, the weight always acts exactly straight down.
Why is this so important to understand?
Well, consider an aircraft flying straight up like a rocket. Bearing in mind all you have just learned…
Which direction are the four forces acting in?
Thrust will be where the aircraft nose is pointing… So, up into the sky. Drag will be acting backward… So down towards the earth. Weight always acts towards the center of the earth, so that too will be pointing backward. And, lift, working at 90-degrees to the upper wing surface, will actually be acting towards the horizon!
This isn’t a good situation to be in. The only force opposing the combined effect of weight and drag is thrust. Unless your aircraft is fitted with a rocket motor, it’s only going to go one way!
Down!
The Effects of the Four Forces of Flight
Now that we understand the basic concepts of the four forces, we can take a look at a few combinations of these forces and hopefully gain a better understanding of how they work together.
They will also be heavily influenced by your actions on the flight controls.
Check out my handy table below to see how they interact in different combinations:
| Force Interaction | What Will Happen to the Aircraft | To Fly Straight and Level |
| Lift Greater Than Weight | Aircraft will climb. | The speed must be reduced or the angle of attack lowered to produce less lift. |
| Weight Greater than Lift | Aircraft will descend. | The angle of attack or speed must be increased to generate more lift. |
| Thrust Greater Than Drag | Aircraft will accelerate. | Thrust must be reduced. Be aware this will result in a loss of lift, meaning you’ll need a higher angle of attack. |
| Drag Greater Than Thrust | The aircraft will decelerate. | Thrust must be increased. Be aware this will cause an increase in lift, meaning you’ll also need a lower angle of attack. |
| Lift, Weight, Thrust and Drag Equally Balanced | Straight and level flight | Do nothing! You are in exactly the right place. |
The Four Forces: Final Thoughts
While the four forces of flight are relatively simple in concept, it is a little bit of a juggling act to understand how they all interact together. Lift is related to speed, as well as drag. Increased weight can mean you need to use more thrust and generate more lift, which leads to more drag, and so on…
It is impossible to consider all of the combinations. Still, you’ll be a much better pilot by understanding the four forces individually and how they work together. When we add flight controls into the mix, the game changes even further. Check this article for all you need to know about flight controls.
Four Fundamentals of Flight (Video)
